core: split Context into a separate grpc-context artifact.

The Context API is not particularly gRPC-specific, and will be used by
Census as its context propagation mechanism.

Removed all dependencies to make it easy for other libraries to depend
on.
diff --git a/core/build.gradle b/core/build.gradle
index b03e06c..6d6147c 100644
--- a/core/build.gradle
+++ b/core/build.gradle
@@ -7,7 +7,8 @@
 dependencies {
     compile libraries.guava,
             libraries.errorprone,
-            libraries.jsr305
+            libraries.jsr305,
+            project(':grpc-context')
     testCompile project(':grpc-testing')
 }